1. Field of the Invention
The present invention relates to an ultrasonic wave diagnostic apparatus or more particularly to an ultrasonic wave diagnostic apparatus of electronic focus type in which an ultrasonic beam is converged by controlling the phase of a signal transmitted from a plurality of piezoelectric elements.
2. Description of the Related Art
In conventional ultrasonic wave diagnostic apparatuses of electronic focus type, the ultrasonic wave beam is converged or deflected by displacing the transmission time of the ultrasonic waves from a plurality of piezoelectric elements and thus controlling the focus of the transmitted waves.
A first conventional ultrasonic wave diagnostic apparatus of this type is disclosed in JP-A-62-277944 (JP-B-6-96016). FIG. 5 shows a configuration of an ultrasonic transmission wave focus control circuit of the conventional ultrasonic wave diagnostic apparatus, and FIG. 6 is a timing chart of an output waveform of the apparatus. The conventional ultrasonic wave diagnostic apparatus, as shown in FIGS. 5 and 6, comprises a delayed clock generating circuit 51 for generating multiphase delayed clock signals CK1 to CK4 and controlling the delay amount of .DELTA.T, a delayed data latch circuit 52 for storing the delay time, a multiplexer 53 for selecting one clock CK0 signal (CK4 in FIG. 6) from the delayed clock signals CK1 to CK4, a delay amount counter 54 for counting the delayed clock signals and determining the delay amount, a transmission wave pulse counter 55 for counting the number of transmission pulses, and a transmission wave pulse generating circuit 56 for generating transmission pulses.
In the conventional ultrasonic wave diagnostic apparatus having the above-mentioned configuration, an output DS of the delayed data latch circuit 52 is counted by the delay amount counter 54 and a signal CR is output. After this signal CR rises, a transmission pulse CP for generating an ultrasonic wave is produced thereby to control the delay amount.
In a second conventional ultrasonic wave diagnostic apparatus suggested in JP-A-62-180267, on the other hand, an oscillation circuit is configured of at least a resistor and at least a capacitor, and the frequency of the transmission pulses is controlled by setting the characteristic values of the resistor and the capacitor.
According to the first conventional ultrasonic wave diagnostic apparatus above mentioned, however, the transmission pulses output from the transmission pulse generating circuit 56 has a frequency obtained by dividing the frequency of the delayed clock signal. For controlling the frequency and the duty factor of the transmission pulses, therefore, an additional circuit is required.
According to the second conventional ultrasonic wave diagnostic apparatus described above, on the other hand, there is a problem that the constants of the resistor and the capacitor are required to be changed for setting the frequency, and therefore there are required resistors and capacitors corresponding to the frequency involved, resulting in the circuit scale being increased in accordance with a number of operating frequencies of the transmission pulses. Further, the dependence of the frequency on the characteristic values of the resistors and the capacitors makes impossible a highly accurate setting. The frequency, therefore, cannot be finely adjusted.